Conventionally, as to methods for producing high purity aluminum according to the segregation principle, there has been known: a method in which the molten aluminum in the vicinity of a solidification interface is stirred such that when molten aluminum is cooled, it is solidified from a downward or circumferential direction; a method in which a cooling body is immersed in the molten aluminum, and the cooling body is rotated while supplying a cooling medium into the cooling body, thereby crystallizing purified aluminum with high purity on the circumferential surface; and the like.
In the purification method according to the segregation principle, the concentrated layer of impurities discharged into the molten aluminum in the vicinity of the solidification interface is reduced in thickness as much as possible to disperse the impurities throughout the molten aluminum. This results in an improvement of purification efficiency. In order to improve purification efficiency as described above at a relatively fast production velocity, there has been proposed a technique whereby the relative velocity between crystallized aluminum and molten aluminum is increased to enhance the efficiency of discharging concentrated impurities into the molten aluminum having less impurities. For example, there has been disclosed a method in Japanese Patent Publication No. 61-3385, wherein a cooling body is rotated so that the relative velocity between the outer regions of the cooling body and the molten aluminum falls in the range of 1600 mm/s to 8000 mm/s, whereby the concentrated layer of impurities in the vicinity of solidification interface is reduced in thickness to enhance the purity of the obtained aluminum. Also, in Japanese Patent Publication No. 63-64504, there has been proposed the following method: in solidifying molten aluminum from a downward direction, bubbles are released from the center of the lower part of a rotating body into the molten aluminum in the vicinity of the solidification interface to induce a concentrated layer of impurities to disperse. This causes the impurities to disperse throughout the molten aluminum, thereby enhancing the purity of the purified aluminum.
Further, as a method for removing hydrogen from molten aluminum with efficiency, there has been proposed a method in Japanese Patent Publication No. 5-852558, in which molten aluminum is stirred, and a gas which can remove hydrogen is blown into the molten aluminum to solidify the molten aluminum in one direction.
However, with these conventional methods, impurities in the obtained aluminum cannot be removed to a sufficient degree. For example, in the method of using a rotated cooling body, it is desirable to make the relative velocity between the outer regions of the cooling body and the molten aluminum as high as possible in order to enhance the purity of the obtained aluminum. However, since the molten aluminum also flows in the same direction in accordance with the rotation of the cooling body, there is a limit on the effect of reducing the concentrated layer of impurities in thickness. Also, even if bubbles are discharged from the center of the lower part of the rotating body into the liquid phase in the vicinity of the solidification interface when the molten aluminum is solidified from a downward direction, the bubbles are lighter in weight than the molten aluminum, and hence move upwards. This limits the arrival at the concentrated layer of impurities in the vicinity of the solidification interface, and the stirring function thereof. Further, even if the molten aluminum is stirred and a gas which can remove hydrogen is blown into the molten aluminum, during which the unidirectional solidification of the molten aluminum is effected, and the hydrogen is removed from the molten aluminum with high efficiency, the removal efficiency of the eutectic crystal impurities such as Si and Fe is not necessarily sufficient.